Generally, data on an optical disc are recorded around a series of concentric circular tracks in a spiral arrangement. For reading data from the tracks of the optical disc, an optical pickup head is driven to see tracks by a sled motor. By using a suitable servo control system, the optical pickup head is properly located above the desired track.
When an electronic signal is generated responsive to an optical signal reflected from the optical disc and received by a four-quadrant photodiode of the optical pickup head, the electronic signal is transmitted to a radio frequency (RF) amplifier to be processed into a radio frequency (RF) signal. By a track-crossing signal generator, the RF signal is then processed into a track-crossing signal. According to a proper slicing level, the track-crossing signal is processed into a sliced track-crossing signal. As the RF signal is changed during track-crossing, the track-crossing signal and the sliced track-crossing signal are changed correspondingly. According to the track-crossing signal and the sliced track-crossing signal, the servo processor performs a track-crossing control operation and a track-following control operation.
FIG. 1 schematically illustrates a circuit diagram of a sliced track-crossing signal generator according to prior art. The sliced track-crossing signal generator principally includes a top envelope detecting unit 10, a bottom envelope detecting unit 20, a track-crossing signal generator 30, a slicing level generator 40 and a comparator 50. A RF signal is simultaneously inputted into the top envelope detecting unit 10 and the bottom envelope detecting unit 20. The top envelope detecting unit 10 detects the top envelope of the RF signal and outputs a top envelope signal. The bottom envelope detecting unit 20 detects the bottom envelope of the RF signal and outputs a bottom envelope signal. The track-crossing signal generator 30 subtracts the bottom envelope signal from the top envelope signal, thereby outputting a track-crossing signal. According to the track-crossing signal, the slicing level generator 40 issues a slicing level. The comparator 50 compares the track-crossing signal with the slicing level, thereby generating a sliced track-crossing signal.
FIG. 2 is a timing waveform diagram illustrating relations between the RF signal, the top envelope signal, the bottom envelope signal, the track-crossing signal, the slicing level and the sliced track-crossing signal. Generally, when the optical pickup head jumps from one track to another track, the bottom envelope signal is varied with change of the focusing position of the laser beam on the optical disc. As shown in FIG. 2, the top envelope signal is substantially kept unchanged. When the laser beam focuses between any two adjacent tracks, the bottom envelope signal has a local maximum value. Whereas, when the laser beam focuses on the track, the bottom envelope signal has a local minimum value. In other words, the bottom envelope signal varies as the focusing position of the laser beam changes.
Please refer to FIGS. 1 and 2 again. The track-crossing signal is generated from the track-crossing signal generator 30 by subtracting the bottom envelope signal from the top envelope signal. The slicing level is generated from the slicing level generator 40 according to the track-crossing signal. The sliced track-crossing signal is outputted from the comparator 50 by comparing the track-crossing signal with the slicing level.
In the servo control system of the optical disc drive, the position of the optical pickup head and the number of jumped tracks are determined according to the sliced track-crossing signal. For example, a pulse of the sliced track-crossing signal is resulted when the optical pickup head jumps from a track to an adjacent track. By counting the pulse number of the sliced track-crossing signal, the number of tracks which are jumped is determined. Moreover, when the sliced track-crossing signal is at the high-level state, the laser beam is focused on the track. On the contrary, the laser beam is not focused on the track when the sliced track-crossing signal is at the low-level state.
As known, due to production failure, fingerprints, dust or scratch, some defects are readily formed on the surface of the optical disc. Due to the defects, the RF signal is deteriorated. The deteriorated RF signal may deteriorate the top envelope signal. According to the deteriorated RF signal, an erroneous sliced track-crossing signal is generated from the comparator 50. Under this circumstance, the servo control system of the optical disc drive fails to accurately realize the position of the optical pickup head and the number of jumped tracks and thus results in track crossing failure.
For addressing the problems described above, a circuit and a method of detecting a mirror signal for an optical disc apparatus have been disclosed in U.S. Pat. No. 6,967,906. The term “mirror signal” used in this publication corresponds to the sliced track-crossing signal described in FIG. 1 and FIG. 2. The sliced track-crossing signal is generated according to the bottom envelope signal without referring to the top envelope signal.
FIG. 3 a circuit diagram of a sliced track-crossing signal generator disclosed in U.S. Pat. No. 6,967,906. As shown in FIG. 3, the sliced track-crossing signal generator principally includes a bottom envelope detector 220, a top holding unit 230, a bottom holding unit 240, a center level detector 250, an amplifier and low-pass filter (AMP & LPF) 260, a comparison voltage determiner 270 and a comparator 280. The bottom envelope detector 220 detects a bottom envelope of the RF signal and outputs a bottom envelope signal. The bottom envelope signal is transmitted to the top holding unit 230, the bottom holding unit 240 and the AMP & LPF 260. The top holding unit 230 detects and holds a top level of the bottom envelope signal and outputs a top holding signal. The bottom holding unit 240 detects and holds a bottom level of the bottom envelope signal and outputs a bottom holding signal.
The center level detector 250 detects a center level of the top holding signal and the bottom holding signal. The comparison voltage determiner 270 can output a comparison voltage, which is controlled to have a level between the top holding signal and the bottom holding signal. When the bottom envelope signal and the center level are received by the AMP & LPF 260, the bottom envelope signal is amplified into an amplified bottom envelope signal. The comparator 280 compares the level of the amplified bottom envelope signal with the level of the comparison voltage and outputs a mirror signal (or a sliced track-crossing signal). In the servo control system of the optical disc drive, the position of the optical pickup head and the number of jumped tracks can be determined according to the mirror signal. Moreover, the comparison voltage can be considered as a slicing level.
FIGS. 4A, 4B, 4C, 4D and 4E are timing waveform diagrams illustrating relations between the RF signal, the bottom envelope signal, the top holding signal, the bottom holding signal, the amplified bottom envelope signal, the center level, the comparison voltage and the mirror signal processed in the sliced track-crossing signal generator of FIG. 3.
As previously described, the RF signal is possibly deteriorated by the some defects (e.g. fingerprint, dust or scratch). As shown in FIG. 4A, the waveform of the top envelope signal is distorted due to the deteriorated RF signal but the influence of the deteriorated RF signal on the bottom envelope signal is negligible.
As shown in FIG. 4B, the bottom envelope signal has a local minimum value when the laser beam is focused on the track. When the laser beam is focused between any two adjacent tracks, the bottom envelope signal is varied with change of the focusing position of the laser beam.
As shown in FIG. 4C, the top holding signal issued from the top holding unit 230 is held at the top level of the bottom envelope signal and the bottom envelope signal issued from the bottom holding unit 240 is held at the bottom level of the bottom envelope signal. The center level is the average of the top level and the bottom level of the bottom envelope signal.
In FIG. 4D, the amplified bottom envelope signal and the comparison voltage are schematically illustrated.
The sliced track-crossing signal generated by comparing the level of the amplified bottom envelope signal with the level of the comparison voltage is shown in FIG. 4E.
The sliced track-crossing signal generator disclosed in U.S. Pat. No. 6,967,906 is effective for improving the track crossing performance of the optical disc deteriorated by the dark defect. This technology, however, fails to be applied to avoid the track crossing failure the optical disc deteriorated by the deep defect or the bright defect.
FIGS. 5A, 5B, 5C and 5D are timing waveform diagrams illustrating relations between the RF signal, the bottom envelope signal, the top holding signal, the bottom holding signal, the center level and the mirror signal processed in the sliced track-crossing signal generator of FIG. 3, in which a deep defect area is present on the disc track.
In a case that the optical pickup head is moved along a track having a deep defect area with no reflection, as shown in FIG. 5A, the RF signal abruptly drops down. That is, when the focusing spot is located on the deep defect area, the RF signal is totally distorted. The bottom envelope signal issued from the bottom envelope detector 220 is shown in FIG. 5B.
As shown in FIG. 5C, the top holding signal issued from the top holding unit 230 is held at the top level of the bottom envelope signal and the bottom envelope signal issued from the bottom holding unit 240 is held at the bottom level of the bottom envelope signal. The center level is the average of the top level and the bottom level of the bottom envelope signal. The sliced track-crossing signal generated from the comparator 280 is shown in FIG. 5D.
As can be seen from FIG. 5C, after the focusing spot passes across the deep defect area, the center level is greatly biased from the normal level and fails to return to the normal level in a short term because the bottom holding signal is suffered from a great variation. As a consequence, the center level and the comparison voltage are suffered from great variations. By using the comparison voltage as a slicing level, the comparator 280 will output erroneous pulses of the sliced track-crossing signal after the focusing spot passes across the deep defect area, as can be seen in FIG. 5D. Under this circumstance, the number of tracks which are jumped is erroneously determined and thus track crossing failure is resulted.
FIGS. 6A, 6B, 6C and 6D are timing waveform diagrams illustrating relations between the RF signal, the bottom envelope signal, the top holding signal, the bottom holding signal, the center level and the mirror signal processed in the sliced track-crossing signal generator of FIG. 3, in which a bright defect area is present on the disc track.
In a case that the optical pickup head is moved along a track having a bright defect area with strong reflection, as shown in FIG. 6A, the RF signal abruptly rises up. That is, when the focusing spot is located on the bright defect area, the RF signal is totally distorted. The bottom envelope signal issued from the bottom envelope detector 220 is shown in FIG. 6B.
As shown in FIG. 6C, the top holding signal issued from the top holding unit 230 is held at the top level of the bottom envelope signal and the bottom envelope signal issued from the bottom holding unit 240 is held at the bottom level of the bottom envelope signal. The center level is the average of the top level and the bottom level of the bottom envelope signal. The sliced track-crossing signal generated from the comparator 280 is shown in FIG. 6D.
As can be seen from FIG. 6C, after the focusing spot passes across the bright defect area, the center level is greatly biased from the normal level and fails to return to the normal level in a short term because the top holding signal is suffered from a great variation. As a consequence, the center level and the comparison voltage are suffered from great variations. By using the comparison voltage as a slicing level, the comparator 280 will output erroneous pulses of the sliced track-crossing signal after the focusing spot passes across the bright defect area, as can be seen in FIG. 6D. Under this circumstance, the number of tracks which are jumped is erroneously determined and thus track crossing failure is resulted.
As previously described, the bottom holding signal or the top holding signal is readily suffered from a great variation when the optical pickup head is moved along a track having a deep defect area or a bright defect area. Since the center level and the comparison voltage are suffered from great variations, the servo control system of the optical disc drive is unstable, which may cause the optical pickup head to have slip track. Eventually, these defects could result in reading or writing errors.
Therefore, there is a need of providing a device and a method for generating a track-crossing signal in an optical disc drive so as to obviate the drawbacks encountered from the prior art.